This invention relates to multiple-drop fiber optic communication systems, and is more particularly directed to a technique for detecting and locating breaks in the feeder fibers in such communication systems.
Recently, fiber-in-the-loop (FITL) architecture has been proposed, and this has been adopted by a number of communications equipment suppliers. The prevalent communications architecture is what has come to be called the Passive Distribution Network (PDN). In this scheme, a feeder fiber leaves the host digital terminal (HDT) to serve up to several hundred subscribers. Each of the subscribers is attached to an optical path by means of an associated opto-electronic interface device, which is referred to as an optical network unit (ONU). In the optical distribution net, branching to the multiplicity of ONUs is accomplished with optical power splitters in the feed net. There can be one or more stages of power splitting. There are no active components in the net between the HDT and ONUs, i.e., there are no active components to fail.
If there is a fiber break in any of the feeders between an HDT and a splitter servicing a number of ONUs, the occurrence of the break can be deduced with good statistical reliability by polling each of the ONUs and mapping their responses. If it is found that a group of ONUs does not respond, it can be assumed that the failure is in one of the feeder fibers. However, if only one individual ONU fails to respond, it is impossible to infer from polling data whether it is the fiber serving that ONU or the ONU itself that is at fault. So long as the question of fiber failure or ONU failure is unresolved, there is a strong possibility of dispatching a repair worker in error to the ONU or to the feeder. As each repair trip involves time and distance to service a remote location, the cost of erroneous dispatch can amount to hundreds of dollars per error.